Our progress this year consisted of a study of the dynamics of schistosome surface antigen expression in vivo and the cloning in an expression vector of several S. mansoni genes encoding antigens recognized by immune mice. A. Surface antigen modulation. While in vivo derived schistosomula express little if any serologically detectable surface antigen, schistosomula maturing in vitro retain their surface antigenicity. This difference was investigated and shown to be due to the rapid shedding of surface antigens when schistosomula are transferred from the in vitro to the in vivo environment. B. Antigen cloning studies. From a series of clones selected from a lambda gt-11 S. mansoni library, two clones were identified which encode antigens recognized by immune mouse antibodies. The antigenicity and immunogenecity of the fusion proteins produced by these clones were characterized.